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CNC sheet metal bending is one of the most underrated processes available for sheet metal part production. With bending, it’s possible to produce a wide variety of part geometries without tooling, at fast lead times, with high levels of repeatability and through automated processes. Bending is especially useful for low and medium volume production, where the reduced quantities (such as, several hundred to several thousand per lot) don’t justify the creation of costly, difficult to maintain stamping tools, or where production costs for other methods are otherwise high for the volume of production required.
Bending techniques are a key tool in the arsenal of product developers, engineers and business owners who are looking to manufacture metal parts. Often, bending is paired with laser cutting as a series of processes to handle low to medium volume production.
It’s good to understand the possibilities with sheet metal bending at the design phase. Bending is a tool that gives engineers the ability to create a wide variety of shapes and designs. In many cases, bending also allows a part to be created from one piece of material. This can have benefits over producing parts from multiple pieces joined together with hardware or welding. These include reducing cost and allowing for improved strength, simplified assembly and little-to-no tooling.
This Komaspec guide provides an overview of the main sheet metal bending processes, the advantages and disadvantages of each, basic design considerations with sheet metal bending and material selection information. This guide, along with our other articles exploring sheet metal fabrication will help you gain a grounding in sheet metal fabrication. The overall aim is to provide you with the information you need to understand how sheet metal parts are manufactured. With this information, you can better discuss the fabrication of your products with sheet metal manufacturers such as ourselves.
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There are multiple ways in which sheet metal parts can be bent during fabrication. However, the two main basic methods are:
The exact process followed with each method will depend on the material being bent as well as the part being produced. Less commonly used methods are employed when bends can’t be achieved through simpler means.
Brake Press
A brake press is a tool that has been in use for many years in traditional fabrication shops all over the world. In its simplest form, a work piece is formed between two dies, as seen in the image below.
Figure 1: CNC Sheet Metal Brake Press (Bystronic Inc.)
Brake presses can be used for a very wide range of sheet and plate materials. Material thicknesses from 0.5mm up to 20mm can be accommodated due to the flexibility of the tooling and the high power levels of hydraulic machinery.
Brake presses are specified by two general parameters: Tonnage and width. The capacity or ‘tonnage’ of a brake press refers to the maximum amount of force it can exert. The material thickness, type and bend radius dictate how many tons of force are needed when fabricating a part. Width refers to the maximum bend length the press can achieve. A typical brake press, for example, could be 100T x 3m (“press brakes”).
Brake press bending processes are categorized into two main types: Air bending and bottom bending.
Air Bending
The most commonly used brake press bending method is air bending. This involves using a brake press with a bottom tool that is a v-shape and a top punching tool of narrow shape with a rounded point. To create a bend, the press pushes the top tool downwards a set distance, bending the material inwards into the the v-shaped bottom tool. Air bending is called air bending because a gap is left between the sheet metal being bent and the bottom tool when the sheet metal is at its full bend depth.
Bottom Bending
Bottom bending also uses a punch and bottom v-shaped die in a brake press. The difference is that the punching tool pushes the sheet metal fully into the die to form a bend that is the shape of the die. The specified bend angle determines the specific die to be used, and so it is necessary to select the correct die for each bend being performed.
Bottom bending generates less springback and creates more accurate angles. However, each bend radius will require a different bottom die, and the process requires more machine pressure. With air bending, many different bend angles can be produced with the same die, less pressure is needed, and the process is faster.
For a full insight into both methods, check out our guide here: Bottom Bending Vs Air Bending.
Figure 2: - Air and Bottom Bending (Skill-Lync)
Rolling
When a cylinder or curved part is required, sheet metal or plate can be rolled to the required curvature. This is achieved with a machine called a roller. Rollers range in size from around 3 feet/1 meter wide to over 5 meters. The thickness of material being bent can range from 1mm to 50mm+.
Figure 3: Bending Rollers (Barnshaws)
The most common rolling machines have 3 rolls, arranged as seen below in figure 4. The middle or top roll is moved closer to the bottom rolls (in some cases vice versa), and the material is then moved through the rollers as they spin. The material deforms as it moves through the rollers, obtaining a curved shape.
As with all bending processes, some springback will occur with rolling. As such, sheet metal parts are generally rolled to a slightly tighter radius than required to compensate for this.
Figure 4: Bending Rollers (Barnshaws)
Once the rolling process is complete, the bottom roller can be adjusted downwards to release the bent section of sheet metal. Otherwise, most rolling machines also have the provision to open the top end yoke as seen below.
One disadvantage, when using rolling to produce a cylinder, is that a pre-bend operation may be required to ensure each end of the cylinder meets after rolling is complete.
Figure 5: End Yoke Removal (“A Rundown on Rolling Machines”)
Sheet metal bending offers a great deal of flexibility in terms of the type and thickness of metals that can be bent. Complex parts can also be produced. Bending processes can be used to create sheet metal parts and assemblies in every industry, including automotive, transport, domestic appliances, furniture, industrial equipment and more.
A wide range of metal types can be bent, including common metals such as steel and aluminum, as well as less common metals, such as copper and titanium. Thick materials can also be bent as well as thin materials. Note that the term ‘sheet metal’ is typically used to refer to materials that are under 3mm in thickness. Sheet metal bending processes, however, can be used on materials that are as thick as 20mm.
In many cases, and with the advent of modern CNC machines that can do both cutting and bending, complete parts can be produced from one piece of sheet metal. Previously, welding or other joining techniques were required where they are now unnecessary. Being able to produce whole parts from one piece of sheet metal can cut costs and production times.
Where it’s not possible to produce a complete part from one piece of material, sheet metal bending can often be combined with other value-adding operations without difficulty. Other fabrication processes, on the other hand, can present issues at this stage.
Mechanical fasteners, such as bolts or more permanent fixings such as rivets or welding, can be used to join bent parts to other parts, for example. Parts of different thicknesses can also be attached to one another as well as parts of the same thickness. Other processes, such as threading, chamfering, countersinking or boring, can also further increase the flexibility and versatility of sheet metal components.
Our article about “Value Added Operations for Sheet Metal Components” provides more information.
Figure 6: Sheet Metal Parts (“Precision Sheet Metal Fabrication and Assembly in China”)
Advantages
Speed of Manufacture - Once designed and programmed, due to the lack of tooling required and the high levels of automation available (many shops are able to run 24/7 with a handful of personnel monitoring production), sheet metal parts can be produced very quickly.
Accuracy - If the considerations that need to be made in the design phase are made adequately, sheet metal parts can be manufactured to a high level of accuracy. Advancements in fabrication techniques and equipment have made it possible to achieve accuracy levels of ±0.05 mm in some cases. As well as bending being accurate in the first place, accuracy can also be repeated consistently. This is particularly true with CNC bending machines with modern software and equipment.
Reduced Post-Processing – Other fabrication processes require post processing before a part is complete. Heat used in welding, for example, can cause dimensional distortion in a sheet metal part. Straightening may be required to correct this. Alternatively, with welding, weld spatter may need to be removed through time-consuming and labor-intensive grinding and polishing. Issues such as these usually aren’t present with bending. Bent sheet metal parts are often ready to go, straight from production.
Less Weight – With sheet metal bending techniques, stiffness and strength can often be achieved in parts without using additional material during manufacture. This reduces part weight and can be beneficial to in-use part performance. This can also help to reduce issues associated with the transport of parts after production.
Low Cost, and Little-to-No Tooling - Due to the advances in technology, using CNC bending processes often cuts down the manual labor required to produce sheet metal parts. As well as less labor being needed, work can also often be performed by unskilled workers rather than more expensive specialist workers.
Because most manufacturers carry a line of common tools (such as punches and dies) that can produce most standard bends, using sheet metal bending processes often eliminates the need for specialized tooling. This means no tooling investment and significantly shorter lead times, as there is no need to wait for complex tooling to be produced, tested or adjusted.
Reduction in Part Complexity – With bending, it’s often possible to create relatively complex components from one piece of material instead of from multiple parts with joints. This reduces time, the potential for errors, failure points and procurement complexity.
Disadvantages
As with all fabrication processes, there are some downsides to using sheet metal bending, as detailed below.
Thickness Limitations – A rule of thumb in sheet metal bending is that thicker materials have higher bend radiuses (“Designing Sheet Metal Components Using Laser Cutting and CNC Sheet Bending”). As a result, tight bends are usually better performed on thinner sections of sheet metal rather than thicker ones.
This can mean that some complex parts can become limited to relatively lightweight materials, suitable for low-load or no-load applications. Bending excessively thick material can also result in the material “bulging” outward post bend (“How Material Properties Impact Air Bending Precision and Tolerances”), the material to crack if it is too rigid, or the need to move to a higher tonnage (and more expensive) press.
Need for Consistent Thickness – Because it’s optimal to produce parts from one piece of material instead of by joining different pieces, it’s better if the thickness of separate flanges on a part does not change. This means that it may be necessary to design a part to have the same thickness throughout.
Cost of Manufacturing - Sheet metal bending is most competitively priced at low to medium volumes. Part volumes from the 100s to 1,000s are usually best. When volumes increase further, stamping is generally considered to be more cost effective, although this can depend on part geometry and other design specifications. This is because CNC bending requires components to be processed one bend at a time, while multiple bends can be produced at the same time through progressive stamping. Even roboticized bending (generally used for volumes of parts in the thousands) cannot compete with high-volume stamping costs.
While labor costs are usually reduced with machine assisted bending processes, in some cases they can be labor intensive. When this is the case, costs will be increased. Some specialist bending projects may also require custom tooling, which while significantly lower than custom stamping tooling, can still be a capital expense.
Production Issues – In some cases, bending will cause indentations or scratches to occur on products during processing, due to the pressure exerted on the part through the narrow bending tool – these types of bending marks are often visible depending on placement in the part. Fractures may also occur if hard metals are bent parallel to the direction sheet metal has been rolled in during production. Holes, slots and other features close to bends can also become distorted during bending. Finally, bends need to be in a position on the sheet metal where there is enough material for it to fit into the equipment without slipping during bending. These issues may all arise during production.
Process Best used for Process Precision Level Thickness (mm) Custom tooling required Minimum order quantity Lead Time from CAD to 1st production
Laser cutting
Small to large parts with every geometry possible
± 0.10mm
0.5mm to 20.0mm
No
1 to 10,000 units
Less than 1 hour
CNC sheet bending
Small to large parts with straight anglegeometry, multiple bend possible
± 0.18mm
0.5mm to 20.0mm
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No
1 to 10,000 units
Less than 1 hour
CNC Punching
Small to large parts with most geometry available, good for parts with multiple holes and embossed
± 0.12mm
0.5mm to 4.0mm*
No unless special form required
1 to 10,000 units
Less than 1 hour
Stamping
High volume production with tight tolerances, restricted geometry
± 0.05 to 0.10mm
0.5mm to 4.0mm*
Yes from 250 USD to 100,000 USD+
≥5,000 units
25 days to 40 days
Shearing
Thin material with simple geometry straight lines and low tolerances requirements
± 0.50mm
0.5mm to 4.0mm*
No
1 to 10,000 units
Less than 1 hour
Table 2: Sheet Metal Bending Compared to Other Fabrication Processes (“Sheet Metal Fabrication”)
Almost all engineering materials are available in sheet form, and thus can be bent to some degree. There are, however, different processing limitations with different materials because of their different inherent properties.
Sheet metal is available in a selection of sizes, which are commonly referred to as gauges. These range from gauge 50 (or 0.03mm), to gauge 1 (7.62mm). Bending with a brake press can be performed with all these thickness gauges and higher (“Sheet Metal Gauge Conversion Chart”).
Gauge is a traditional term still widely used, despite many materials, such as steel and stainless steel, being specified directly in their millimeter thicknesses. This is especially the case in Europe. One exception is aluminum, which is often still defined in all three dimensions by imperial measurements, i.e. feet and inches, and gauge for thickness.
For the best information on the materials available, refer to our standard materials page.
Fig 7. Sheet Metal Parts (“Sheet Metal Surface Finishing Standard Options”)
Each metal has its own unique characteristics, and the following table outlines some of the factors you should consider when making your choice of materials.
MATERIAL SURFACE FINISH YIELD (MPA) TENSILE (MPA) HARDNESS GB STANDARD Powder Coating E-Coating Zinc Plating Darcomet Anodized PassivationCold Rolled Steel (CSR)
SPCC ✓ ✓ ✓ ✓ ≥210 ≥350 HB 65 - 80 JIS G3141-2009 SAPH440 ✓ ✓ ✓ ✓ ≥305 ≥440 HB 80 ± 30 Q/BQB 310-2009Hot Rolled Steel
Q235 ✓ ✓ ✓ ✓ ≥235 375 - 500 HB 120 ± 40 GB/T 700-2006 Q345 ✓ ✓ ✓ ✓ ≥345 GB/T 490 - 675 HB 120 ± 40 1591-2008Spring Steel
65Mn ✓ ✓ ≥785 ≥980 HB 190-340 GT/T 1222-2007Aluminum
AL1060 ✓ ✓ ≥35 ≥75 HB 26 ± 5 GB/T 3190-2008 AL6061 T6 ✓ ✓ ≥276 ≥260 HV 15 – 18 GB/T 3190-2008 AL6063 T5 ✓ ✓ ≥170 ≥250 HB 25 ± 5 GB/T 3190-2008 AL5052 H32 ✓ ✓ ≥70 210-260 HB 11 ± 2 GB/T 3190-2008Stainless Steel
SS301 ✓ ✓ ≥205 ≥520 HB 76 – 187 GB/T 8170-2008 SS304 ✓ ✓ ≥205 ≥520 HB 76 – 187 GB/T 24511-2009 SS316 ✓ ✓ ≥205 ≥520 HB 76 – 187 GB/T 24511-2009Cold Galvanized Steel
SGCC ✓ ≥200 ≥380 HB 60 - 65 JIS-G3302Table 3. Sheet metal materials (“Sheet Metal Standard Options in China”)
Mild Steel - This is available in both hot and cold rolled variants. Both offer excellent cold working performance, with high ductility. Also known as low carbon steel, mild steel is the most commonly used material in the world (“5 Most Popular Types of Metals and Their Uses”).
The largest downside to mild steel is the requirement for coating, which is needed to prevent rust from forming in the presence of moisture. Galvanized steel is available to counter this issue. This comes with a hard wearing pre-applied zinc coating that prevents rust.
Aluminum - First used for aircraft production, various aluminum alloys are available, with a very wide range of applications. Because aluminum alloys with other elements so successfully, an incredibly wide range of types of aluminum alloy can be sourced. These come with a range of different properties.
The most used aluminum alloys for sheet metal applications are the 1000 series alloys, particularly 1060 aluminum. This alloy is widely used due to its high workability and low weight. The 6000 series is also widely used in sheet metal bending. The high level of workability in these metals allows the material to be bent to tight radii without cracking, specifically, and this is often vital for complex parts (“Aluminium / Aluminium 1060 Alloy”).
For general guidelines to material suitability for CNC bending, see the table below:
MATERIAL MALLEABILITY 6061 Aluminum Difficult to bend and often cracks. Cold bending will weaken the metal. Annealing improves malleability. 5052 Aluminum Very malleable and a good choice when using aluminum. Cracking is rare unless a part is reworked. Annealed Alloy Steel Varies based on the alloy. 4140 has good malleability. Annealing helps prevent cracking. Brass Zinc content is important. Higher zinc levels make it less malleable. Good for simple bends but complex parts may require heat. Bronze More difficult to bend and may require heat to avoid cracking. Copper Very malleable. Cold Rolled Steel Less malleable than hot rolled steel. Hot Rolled Steel More malleable than cold rolled steel. Mild Steel Very malleable. Heat not required. Spring Steel Malleable when annealed. Once work hardened, it requires heat to bend again. Stainless Steel Stainless steels like 304 and 430 are easier to form than 410, which can be brittle. Different grades will perform differently although stainless steel is prone to work hardening. Titanium Strong material, so best to design with a large internal bend radius. Overbending required because of springback.Table 4: Material Properties (“How Material Properties Impact Air Bending Precision and Tolerances”)
Stainless Steel - Commonly used in the food and medical industries, stainless steel is an alloy of mild steel. To be stainless steel, steel must contain over 10.5% chromium. This gives the material corrosion resistance, with some grades excelling at resistance to acids, alkalis and other chemicals.
Commonly used grades of stainless steel are 301, 304 and 316, with the latter having higher strength and corrosion resistance, 301 having superior flexibility and “spring” and 304 being a good middle of the road material for general use (Burnett).
Parts that are to be processed using bending equipment should be designed from the outset with the characteristics and limitations of the bending process in mind (“Designing Sheet Metal Components Using Laser Cutting and CNC Sheet Bending”).
We will discuss the main considerations that need to be made below. For even more information, however, you can refer to our sheet metal design guide.
Bend Radius - When a material is formed into a bend, the outer surface is stretched, and the inner surface is compressed. The result is that the part has a rounded corner at the bent edge on both the inside and outside.
The bend radius is a measurement of the curvature of the inside bend edge. The bend radius that is possible with a section of sheet metal will differ depending on the material being bent as well as the tooling geometry and material condition.
It’s good practice to ensure that all bends on a particular part are equal in radius because this greatly simplifies tooling set up, reducing cost.
Bend Length - Another critical pressing variable is bend length. The bend length required will usually depend on the design specifications of a sheet metal part. Bending machines, however, all have maximum widths according to their physical size and configuration. It’s best to seek guidance if your parts are above 2 m as this is a standard sheet and press brake size.
Bend to Bend Distance - When making bends, a physical limit on how close bends can be together is enforced by the size and shape of the tooling being used in the bending machine. Bends on the same side of a metal sheet that are too close will interfere with the tooling, and bends on opposing sides will often be impossible to reach because of the bottom tool.
Common instances of this occurring are where ‘U’ sections are required with the legs or upright flanges being longer than the horizontal section. In some cases, extra deep tooling can be used.
If your bends do need to be close together, it may be possible to find workaround solutions. Alternatively, it may be possible to implement supplementary processes, such as welding or bolting to get to the correct geometry.
Hole to Edge Distance - When bends are produced, the material is stretched. This causes internal stresses that are evenly distributed across the part. If a hole or slot is made too close to a bend, these stresses will be focused on this hole, and this could cause deformation.
Springback - Metals have elasticity and will tend to return towards their original position to a small degree after bending. This effect is called ‘springback’. The exact process is related to metal’s compressive and tensile strength. After bending, sheet metal is compressed on the inside, where the press is applied, and stretched on the outside. Because the material has a higher compressive strength than tensile strength, it springs back towards its original shape.
In practice, springback generally only amounts to 1-2°. This can often be sufficiently compensated for in brake press control because many sheet metal parts don’t need a high level of accuracy. The latest CNC bending machines even incorporate built-in sensors and control to automatically compensate for material variability and other factors to ensure consistent performance.
Where a high level of accuracy is needed, springback can be a challenge because it can be difficult to accurately calculate how much there will be.
Several factors affect springback:
Fabricators often use the K-factor to calculate springback and better understand how to compensate and achieve tighter tolerances where accuracy is needed.
Pressing Tolerances - As with any fabrication process, there are tolerances on dimensional accuracy. These often arise due to variations in sheet metal composition, thickness and processing. Variation should be considered when designing parts, and each process should be utilized to its strengths according to the material being used and part specifications.
CNC control has reduced variation in recent years, and most tolerances can be achieved with modern press brake machines. It can, however, still be a pertinent issue, particularly when designing complex or precision parts.
Here are some rules of thumb:
Consulting with an experienced sheet metal fabricator can help you better understand tolerances in sheet metal bending processes.
Tonnage - Factors such as bend radius, material properties, material type and bend length all contribute to how much pressure is required to make a particular bend. As mentioned, presses have a maximum tonnage capacity, and it may be worth checking that it will be possible to perform the bends you need. Check with your manufacturer before committing to a design you are unsure of.
Heat Affected Zones (HAZ) - Processes such as laser and plasma cutting create heat affected zones in metal. These can sometimes cause issues during bending, such as inconsistent bending near holes and edges. Another issue sometimes seen is cracking due to the increased surface hardness from cutting. If your parts will need other processes that create heat, these issues may need to be taken into consideration.
Sheet Bending Linear Bend Angle Standard High Precision Standard High Precision ±0.1mm ±0.05mm ±1˚ ±0.5˚Table 5: CNC Sheet Metal Bending - Process Tolerances and Techniques
Feature Type Minimum Distance Guidelines* Between a curl and an internal bend ±6 times the curl's radius plus the thickness of the material Between a curl and an external bend ±9 times the curl's radius plus the thickness of the material Between a hem and an external bend ±8 times the sheet thickness Between a hem and an internal bend ±5 times the sheet thickness Between a counterbore and a bend ±4 times the sheet thickness plus bend radius Between a countersink and a bend ±3 times the sheet thickness Between hole and a bend ±2.5 times the material length + bend radius Between a slot and a bend ±4 times the sheet thickness plus the bend radius Between an extruded hole and a bend ±3 times the sheet thickness plus the bend radius Between semi-pierced hole and a bend ±3 times the sheet thickness plus the bend radius Between a notch and a bend in a perpendicular plane ±3 times the sheet thickness plus the bend radius Between a notch and a bend in a parallel plane ±8 times the sheet thickness plus the bend radius Between a dimple and a bend ±2 times the sheet thickness plus inside radius of the dimple plus the bend radius Between rib to a bend perpendicular to the rib ±2 times sheet thickness plus the radius of the rib plus the bend radius
Table 6: Minimum Distance Guidelines for Features Relative to Bends
Sheet metal bending has distinct advantages over alternative sheet metal fabrication processes, including higher output, lower cost and high flexibility in design. It also removes many difficulties associated with assembly techniques such as welding or riveting. With careful consideration during the design process, and with the aid of modern technology, sheet metal parts can be made stronger, lighter and more quickly through bending than through traditional fabrication methods.
The sheet metal specialist at Komaspec are happy to work to review your product design together and to help you select the fabrication process that best suits your product design and application needs.
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